window.addEventListener('load', function() {
    let llis = document.querySelector('#left').querySelectorAll('li');
    let rlis = document.querySelector('#right').querySelectorAll('li');
    let clis = document.querySelector('#center').querySelectorAll('li');
    // console.log(llis);

    for (let i = 0; i < llis.length; i++) {
        llis[i].addEventListener('mouseenter', function() {
            for (let i = 0; i < clis.length; i++) {
                clis[i].style.display = 'none';
            };
            clis[i].style.display = 'block';
        });
        rlis[i].addEventListener('mouseenter', function() {
            for (let i = 0; i < clis.length; i++) {
                clis[i].style.display = 'none';
            };
            clis[i + 9].style.display = 'block';
        })
    }
})